1. Field of the Invention
The present invention relates to improved thermoelectrics for producing heat and/or cold conditions with a greater efficiency.
2. Description of the Related Art
Thermoelectric devices (TEs) utilize the properties of certain materials to develop a thermal gradient across the material in the presence of current flow. Conventional thermoelectric devices utilize P-type and N-type semiconductors as the thermoelectric material within the device. These are physically and electrically configured in such a manner that the desired function of heating or cooling.
Some fundamental equations, theories, studies, test methods and data related to TEs for cooling and heating are described in H. J. Goldsmid, Electronic Refrigeration, Pion Ltd., 207 Brondesbury Park, London, NW2 5JN, England (1986). The most common configuration used in thermoelectric devices today is illustrated in FIG. 1. Generally, P-type and N-type thermoelectric elements 102 are arrayed in a rectangular assembly 100 between two substrates 104. A current, I, passes through both element types. The elements are connected in series via copper shunts 106 soldered to the ends of the elements 102. A DC voltage 108, when applied, creates a temperature gradient across the TE elements. FIG. 2 for flow and FIG. 3 for an object both illustrate general diagrams of systems using the TE assembly 100 of FIG. 1.
When electrical current passes through the thermoelectric elements, one end of the thermoelectric elements becomes cooler and the other end becomes warmer. TE's are commonly used to cool liquids, gases and objects.
The basic equations for TE devices in the most common form are as follows:
                              q          c                =                              α            ⁢                                                  ⁢                          IT              c                                -                                    1              2                        ⁢                          I              2                        ⁢            R                    -                      K            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            T                                              (        1        )                                          q          in                =                              α            ⁢                                                  ⁢            I            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            T                    +                                    I              2                        ⁢            R                                              (        2        )                                          q          h                =                              α            ⁢                                                  ⁢            I            ⁢                                                  ⁢                          T              h                                +                                    1              2                        ⁢                          I              2                        ⁢            R                    -                      K            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            T                                              (        3        )            where qc is the cooling rate (heat content removal rate from the cold side), qin is the power input to the system, and qh is the heat output of the system, wherein:
α=Seebeck Coefficient
I=Current Flow
Tc=Cold side absolute temperature
Th=Hot side absolute temperature
R=Electrical resistance
K=Thermal conductance
Herein α, R and K are assumed constant, or suitably averaged values over the appropriate temperature ranges.
Under steady state conditions the energy in and out balances:qc+qin=qh  (4)Further, to analyze performance in the terms used within the refrigeration and heating industries, the following definitions are needed:
                    β        =                                            q              c                                      q              in                                =                      Cooling            ⁢                                                  ⁢            Coefficient            ⁢                                                  ⁢            of            ⁢                                                  ⁢            Performance            ⁢                                                  ⁢                          (              COP              )                                                          (        5        )                                γ        =                                            q              h                                      q              in                                =                      Heating            ⁢                                                  ⁢            COP                                              (        6        )                                          From          ⁢                                          ⁢                      (            4            )                          ;                                                                                                q              c                                      q              in                                +                                    q              in                                      q              in                                      =                              q            h                                q            in                                              (        7        )                                          β          +          1                =        γ                            (        8        )            So β and γ are closely connected, and γ is always greater than β by unity.
If these equations are manipulated, conditions can be found under which either β or γ are maximum or qc or qh are maximum.
If β maximum is designated by, βm, and the COP for qc maximum by, βcm, the result is as follows:
                              β          m                =                                            T              c                                      Δ              ⁢                                                          ⁢                              T                c                                              ⁢                      (                                                                                1                    +                                          ZT                      m                                                                      -                                                      T                    h                                                        T                    c                                                                                                                    1                    +                                          ZT                      m                                                                      +                1                                      )                                              (        9        )                                                      β            cm                    =                                    (                                                                                          1                      2                                        ⁢                                          ZT                      c                                                        -                                      Δ                    ⁢                                                                                  ⁢                    T                                                                                        ZT                    c                                    ⁢                                      T                    h                                                              )                        ⁢                                                  ⁢            where                          ;                            (        10        )                                Z        =                                            α              2                        RK                    =                                                                      α                  2                                ⁢                ρ                            λ                        =                          Figure              ⁢                                                          ⁢              of              ⁢                                                          ⁢              Merit                                                          (        11        )                                                      T            m                    =                                                                      T                  c                                +                                  T                  h                                            2                        ⁢                                                  ⁢            and                          ;                            (        12        )            Wherein:
λ=Material Thermal Conductivity; and
ρ=Material Electrical Resistivity
Note that for simple solid shapes with parallel sides, K=λ×area/length. Similarly R=(ρ×length)/area. Thus, any change in shape, such as a change in length, area, conality, etc., can affect both K and R. Also, if the shapes of flexible elements are changed by mechanical or other means, both K and R can change.
βm and qcm depend only on Z, Tc and Th. Thus, Z is named the figure of merit and is basic parameter that characterizes the performance of TE systems. The magnitude of Z governs thermoelectric performance in the geometry of FIG. 1, and in most all other geometries and usages of thermoelectrics today.
For today's materials, thermoelectric devices have certain aerospace and some commercial uses. However, usages are limited, because system efficiencies are too low to compete with those of most refrigeration systems employing freon-like fluids (such as those used in refrigerators, car HVAC systems, building HVAC systems, home air conditioners and the like).
The limitation becomes apparent when the maximum thermoelectric efficiency from Equation 9 is compared with Cm, the Carnot cycle efficiency (the theoretical maximum system efficiency for any cooling system);
                                          β            m                                C            m                          =                                                                              T                  c                                                  Δ                  ⁢                                                                          ⁢                  T                                            ⁢                              (                                                                                                    1                        +                                                  ZT                          m                                                                                      -                                                                  T                        h                                                                    T                        c                                                                                                                                                1                        +                                                  ZT                          m                                                                                      +                    1                                                  )                                                                    T                c                                            Δ                ⁢                                                                  ⁢                T                                              =                      (                                                                                1                    +                                          ZT                      m                                                                      -                                                      T                    h                                                        T                    c                                                                                                                    1                    +                                          ZT                      m                                                                      +                1                                      )                                              (        13        )            Note, as a check if Z→∞, β→Cm. The best commercial TE materials have Z such that the product;ZTa≈1
Several commercial materials have a ZTa=1 over some narrow temperature range, but ZTa does not exceed unity in present commercial materials. This is illustrated in FIG. 4. Some experimental materials exhibit ZTa=2 to 4, but these are not in production. Generally, as better materials may become commercially available, they do not obviate the benefits of the present inventions.
Several configurations for thermoelectric devices are in current use for automobile seat cooling systems, for portable coolers and refrigerators, for high efficiency liquid systems for scientific applications, for the cooling of electronics and fiber optic systems and for cooling of infrared sensing system.
All of these devices have in common that the Th is equalized over the hot side of the TE, and similarly, Tc is equalized over the cold side. In most such devices, the TEs use an alumina substrate (a good thermal conductor) for the hot and cold side end plates and copper or aluminum fins or blocks as heat exchangers on at least one side.
Thus, to a good approximation, conditions can be represented by the diagram in FIG. 5. In this case ΔT has been split into the cold side at ΔTc and hot side ΔTh whereΔT=ΔTc+ΔTh.Using (1) and (2) in (5):
                    β        =                                            q              c                                      q              in                                =                                                    α                ⁢                                                                  ⁢                                  IT                  c                                            -                                                1                  2                                ⁢                                  I                  2                                ⁢                R                            -                              K                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                T                                                                    α                ⁢                                                                  ⁢                I                ⁢                                                                  ⁢                Δ                ⁢                                                                  ⁢                T                            +                                                I                  2                                ⁢                R                                                                        (        14        )            
But ΔT is the sum of ΔTc and ΔTh. So, for example, if ΔTc=ΔTh then ΔT=2ΔTc. Since the efficiency decreases with increasing ΔT, it is highly desirable to make ΔT as small as possible. One option is to have the fluid flowing by the hot side be very large compared to that by the cold side. For this case, the equation for heat flow from the hot side is:qh=CpMΔTh  (15)where CpM is the heat capacity of the fluid passing the hot side per unit time (e.g., per second).
Thus, if CpM is very large for a given required qh, ΔTh will be very small. However, this has the disadvantage of requiring large fans or pumps and a large volume of waste fluid (that is, fluid not cooled, but exhausted as part of the process to achieve more efficient cooling).
A second option is to make the heat sink on the hot side very large so that the heat is dissipated passively. Examples would be a low power TE in a car with the hot side in very good thermal contact with the vehicle chassis, or a TE system in a submarine with the TE in good thermal contact with the hull and hence, the ocean water. In general, however, these methods are difficult to implement or cost, weight, size or other conditions limit their use. The result is that ΔT is substantially larger than ΔTc in most devices, and efficiency suffers accordingly.